added marshal_load and marshal_dump for ProxyTestResult. Behavior of met...

Ruby-2.0.0 later changed behaivior of method_missing. method_missing of any Object repond to marshal_dump and marshal_load when call with Marshal.dump and Marshal.load.

details of this behaivior discussion:
 * http://bugs.ruby-lang.org/issues/7564
 * http://bugs.ruby-lang.org/issues/7638

I think this problem is that ProxyTestResult calling marshal_load and marshal_dump but don't have these methods. I added it.

@tenderlove How do you think about this problem?